1. Field of the Invention
This invention relates to sound generators and more particularly to an apparatus and method for the generation of high-intensity "directionally perceptible" (i.e., locationally discernible) sound. In one application, the sound generator hereof is used in a personal alert safety system to facilitate the location of lost personnel where it is important that a human determination be quickly made as to the direction to the sound source.
2. Description of the Related Art
Some sound sources appear to have better directionality than other sound sources. Typical examples of sound sources with good directional characteristics are foghorns and air actuated horns frequently used on large trucks. Such devices are large in size, heavy in weight and require large power inputs and are, therefore, unsuitable in many applications. Additionally, clapper, vibrating, or diaphragm horns, typical of electric auto horns or bicycle horns, have some direction qualities. However, these horns also have objectional electrical characteristics related to power conversion and inductive circuits which complicate their use.
Piezoelectric transducers in sound generating devices are power efficient, small in size, and light in weight, and when mounted in suitable resonant chambers are capable of generating high intensity sound at low power inputs. The resonant frequency of such piezoelectric transducer is, however, high, typically on the order of 3000 Hz and above, and appear to have little directionality of sound, at least with human beings.
Several different techniques have evolved to utilize piezoelectric transducers as sound generators. These techniques include amplitude modulation of the oscillating power signal energizing the resonator at a rate between about 200 and 800 Hz, so that the sound output of the generator is amplitude modulated at that rate. Such devices require a discrete oscillator in an external drive configuration to generate the signals for exciting the resonator for high sound levels. Environmental stability of the resonator and of the decoupled driving oscillator with an amplitude modulated output, are not easily achievable, nor is it easy to match the resonator and oscillator frequencies in the first place. Mismatching of resonator and oscillator frequencies results in a serious diminution of sound intensity.
Another solution is to utilize wide band sound transducers which are relatively insensitive to drift in the frequency of the energizing source. However, wide band sound transducers are not resonant and therefore are inefficient, requiring relatively large power inputs for comparable sound intensity.
Still another technique is to utilize two or more resonant transducers or a single resonant transducer having multiple resonant frequency outputs. The generation of two sound sources of relatively high nondirectional frequency creates a beat frequency which may be directionally perceptible. The most conspicuous disadvantage of using beat frequency devices is that only approximately 25 percent of the radiated sound is converted into the directionally perceptible lower frequency beat. Not only is the majority of the sound wasted for directional purposes, but it tends to overpower the beat note. See e.g., U.S. Pat. No. 4,486,742.